Process and agent for the detection of silver ions

ABSTRACT

An indicator for the colorimetric detection of silver ions is formed by impregnating an absorbent carrier with an intimate mixture of cadmium sulfide and selenium.

O United States Patent [151 3,661,532

Schmitt et al. 51 May 9, 1972 54 PROCESS AND AGENT FOR THE 58 Field of Search ..252/408; 23/253 TP DETECTION OF SILVER IONS 56 R i C'ted 721 Inventors: Dieter Schmitt; Alfred Stein; Wilhelm I 1 e Baumer, all of Darmstadt, Germany UNITED STATES PATENTS 1 Assignee= Merck Pate"! Gesellschafl 3,510,263 5/1970 l-lach ..23/253 TP beschrankter Haftung, Darmstadt, Germany Primary Examiner-John T. Goolkasian {22] Filed: Dec. 9, 1970 Assistant Examiner-Lorraine T. Kendell 1 pp NO: 96,665 Attorney-Mfllen, Raptes & Whlte 57 ABSTRACT [30] Foreign Application Priority Data An lndlcator for the colorlmetrlc detectlon of silver lons ls Aug. 4 l970 Germany 20 33 1 formed by impregnating an absorbent carrier with an intimate mixture of cadmium sulfide and selenium. [52] U.S. Cl. ..23/253 TP, 23/230 [51 Int. Cl ..C09k 3/00, GOln 33/00 13 Claims, No Drawings PROCESS AND AGENT FOR THE DETECTION OF SILVER IONS BACKGROUND OF THE INVENTION This invention relates to a novel process, composition and agent for the colorimetric determination of silver ions in solution.

The high and rising price of silver makes it increasingly important to recover silver from various waste products, e.g., the waste products from film processing. It is particularly profitable to recover silver from consumed and spent fixing baths. It is therefore very important to be able to provide processes and agents to monitor recovery plants and fixing baths in order to detect the presence of silver therein and semiquantitatively determine the amount of silver in the fixing baths.

it is also important to be able to determine the functional power of photographic fixing baths in a simple manner, so that properly developed films and papers will be obtained therefrom. It is also desirable to be able to determine the silver content of fixing baths in fixing equipment in a simple manner to ensure during continuous operation that at all times the bath contains the technically most advantageous silver concentration.

The use of a test paper which can be employed for these purposes is known. However, this paper still exhibits a number of drawbacks. For example, yellow test paper containing cadium sulfide, attains a darker shade only gradually after immersion into a used fixing bath. Moreover, a definite final color does not occur within a practicable period of time. Thus, the inability to obtain a constant color within a short period of time makes it very difficult to determine the silver content ofa fixing bath with the aid of a color scale. Besides, this conventional semiquantitative silver determination is conducted preferably in the presence of the light of an incandescent lamp. Another disadvantage is that when a conventional test paper is immersed into a fixing bath containing a low silver concentration, e.g., about l-200 mg./l. of Ag(I), the presence of the silver ions is indicated by the darkening ofthe test paper several minutes after immersion therein.

It has now been discovered that these disadvantages can be avoided with the process and agents for detecting the presence of silver ions in a solution of this invention. By employing the novel process and novel agents of this invention, it is now possible to obtain the maximum color depth based upon the silver ion concentration of a solution, e.g., fixing baths, in a very short period of time. The color produced thereby remains constant for a long period so that a comparison with a color scale is readily possible. Thus, it is possible to conduct semiquantitative tests in order to determine, with ease, the amount of silver ions in a solution. In addition, the test is capable of being used by those not skilled in laboratory techniques.

The novel detection reaction of this invention exhibits particular advantages for the qualitative and semiquantitative determination of silver ions present in solutions, including fixing baths. By means of the fixing bath test, a quick and reliable determination can be made in photo and X-ray laboratories in order to determine whether the fixing bath has sufficient residual fixing power or must be replaced by a new bath. The capacity of normal fixing baths containing sodium thiosulfate, as experience has shown, requires the following concentration: 2 g. of Ag(l) per liter for papers, 3 g. of Ag(l) per liter for films, and 4 g. of Ag(l) per liter for X-ray films. The capacity of rapid fixing baths containing ammonium thiosulfate, is, for papers, films and X-ray films, approximately g. of Ag(l) per liter.

With the aid of the novel process and the novel composition, it is also possible to detect with certainty silver ion con centrations as low as 10-100 p.p.m. Thus, the novel detection test of this invention is also more sensitive than the conventional processes known heretofore.

SUMMARY OF THE INVENTION This invention relates to a process, composition and agent for the colorimetric detection of silver ions in a solution comprising an admixture of cadmium sulfide and finely dispersed selenium.

DETAILED DISCUSSION The compositions of this invention comprise an admixture of cadmium sulfide and finely divided selenium. For best results, the selenium and cadmium sulfide are present in the novel compositions of this invention in a weight ratio of less than one, e.g., about 1:3 to about 1:75, preferably about 1:8 to about 1:40. According to a preferred embodiment of this invention, the compositions are applied to an absorbent carrier, thus facilitating the easy use thereof. The cadmium sulfide and selenium particles are in intimate admixture, i.e., each particle of selenium is preferably in physical contact with one or more particles of cadmium sulfide.

A particularly suitable agent for detecting the presence of Ag-lions is obtained when cadmium sulfide is produced in the presence of finely dispersed, particulate, selenium. The selenium is preferably obtained from selenious acid (H SeO or a salt thereof by reduction.

Sodium selenite (Na SeO is preferred. The reduction can be easily carried out by employing sodium sulfide as the preferred reducing agent. When sodium sulfide is employed, the sulfide ions can also be used for producing the cadmium sulfide. However, other reducing agents which are capable of converting selenites into selenium can also be utilized, e.g., hydroxylamin hydrochloride, hydrazine sulphate. A particularly suitable reducing agent is ascorbic acid.

The finely dispersed selenium can be formed simultaneously with the CdS; it can be mixed in finely divided form with the CdS during the preparation of the CdS; or mixed with the CdS after the formation thereof. Preferably, the selenium is admixed with the CdS in a very finely divided form, more preferably in combination with a protective colloid, for example, celluloses, colloids, e.g., hydroxypropylcellulose, polyvinylpyrrolidone or polyvinyl alcohols, being added thereto. It is essential that the selenium and cadmium sulfide be intimately and uniformly intermixed. Thus, for example, a water-soluble cadmium salt can be reacted with sodium sulfide in the presence of very finely dispersed selenium so that the cadmium sulfide intermixes with the selenium as it is formed. The particle size of the selenium can range from about 1 ,u. 60 ,u, preferably about 5 p. 20 pt. While the exact particle size of the CdS and Se is not critical, the finer their particle size, the more sensitive is the resulting admixture as a colorimetric reagent.

The cadmium sulfide is suitably produced by reacting a water-soluble cadmium salt with a water-soluble sulfide, e.g., Na S, K 8, Li S, H 8, preferably sodium sulfide, (preferably Na s 9H O). Examples of suitable water-soluble cadmium salts include cadmium bromide, chloride, acetate, nitrate and/0r sulfate. It is only necessary to add an equivalent amount of sulfide ions, calculated on the cadmium ions present.

A test paper can be prepared from the composition of this invention by impregnating an absorbent paper with an aqueous solution of a water-soluble cadmium salt, e.g. cadmium chloride, and selenious acid or a salt thereof, e.g., sodium selenite, in an amount at least sufficient to provide the desired amount of selenium in the final composition. The impregnated paper is dried and then impregnated with an aqueous solution of sodium sulfide in an amount sufficient to reduce the selenuous acid of salts thereof already present to the desired selenium and convert the water-soluble cadmium salt to CdS. The sodium sulfide can also be dissolved in methanol, a watermethanol mixture or other aqueous organic solvent mixture. Preferred impregnating solutions contain, for example, in ml. of water about 0.3 5 g. CdCl or molar equivalent amount of another soluble cadmium salt and 0.01 0.5 g.

H SeO or molar equivalent amount of Na,SeO or other soluble salt of selenous acid. The second impregnating solution preferably contains 0.3 6.0 g. of Na s 91-1 in 100 ml. of water, methanol or a mixture thereof.

A test paper can also be obtained by whipping pure paper linters or cellulose fibers by vigorous agitation in an aqueous solution comprising a water soluble cadmium salt, e.g., CdCl and H,Se0;, or a salt thereof, e.g., Na SeO With vigorous stirring, a solution of a water-soluble sulfide, e.g. Na s 91-1 0 in water or methanol is then added dropwise thereto resulting in the formation of a yellow CdS-Se co-precipitate. This precipitate is deposited in a very finely particulate form on the fibers. The latter can thereafter be processed into a paper according to conventional methods. A highly suitable indicator paper is formed from absorbent filter paper having a weight per unit area of 100-140 g./m which contains, for example, the following reagents per square meter: 0.35 7 g. Cd (equivalent to 0.45 9 g. C(15) and 0.01 0.8 g. Se.

The ratio of Se to Cd is preferably about 1:10 to 1:50 and more preferably 1:20 to 1:25.

Instead of forming papers, suspensions or pastes of the novel CdS-Se compositions can be used for the detection of silver ions in the same manner.

When impregnated papers are employed, they can either be cut into convenient strips or preferably into square pieces which can be glued onto any convenient supporting substitute, e.g., plastic films, paper or metallic strips or they can be sealed onto or into such films or strips.

Employing the novel process and the novel compositions, Ag(l) can be detected with certainty at a concentration of from 10 to 100 mg./l. with the conversion of the light yellow composition to a dark yellow color. At higher silver ion concentrations, the color becomes brown to black. The color change occurs within seconds after the novel composition is contacted with the Ag(l) solution and remains constant for a long period thereafter. Even when there are present very small amounts of silver, e.g., as low as 10 p.p.m., a perceptible color change can clearly be observed within 1-2 minutes after initiating the detection reaction.

When employing the known indication reactions, 21 comparable color contrast can only be attained after minutes, when a solution containing 100 ppm. of Ag+ is being tested, Another advantage of the invention which becomes readily apparent is that the novel detection procedure can be conducted semiquantitatively. In addition, clear differentiations are possible among Ag(l) solutions of 0.5, l, 1.4, 2, 3, 6, and g./l. concentration. The determination is not impaired by the presence of iron ions. Also any excess selenious acid surprisingly does not have an adverse effect on the colorimetric reaction.

When a test solution contains a very low quantity of silver, e.g., below 500 ppm, it is advantageous to conduct the detection reaction in the presence of at least traces of light. Daylight is usually sufficient, even brief exposure thereto. The detection reaction can also advantageously be conducted in the presence of UV radiation of fluorescent bulb irradiation. The detection reaction can be carried out at a mildly acid to neutral pH, i.e., about 3-7.

Without further elaboration, it is believed that one skilled in the art can, using the preceding description, utilize the present invention to its fullest extent. The following preferred specific embodiments are, therefore, to be construed as merely illustrative, and not limitative of the remainder of the disclosure in any way whatsoever.

EXAMPLE 1 Filter paper (Schleicher and Schiill No. 1451 or No. 2316, weight per unit area 120 g./m is impregnated with a Solution 1 (CdCl H SO set forth in the table below, dried, then impregnated with a Solution 2 (Na s) set forth in the table below and once again dried. The amounts disclosed are, in each case, dissolved in, respectively, 100 ml. of water. The thus-obtained yellow colored impregnated papers are either cut into strips (preferably about 6X60 mm.) or divided into small squares (about 5X5 mm.) The latter are, in turn, glued onto plastic strips of the above-mentioned dimension. Suitably, the reaction zones are cemented at a spacing of about 1-5 mm. from the lower rim of the plastic strips so that the test rods can be conveniently handled at their upper end. The test rods are dipped for about 1 second into the solution to betested, in such a manner that the yellow indicator zone is completely wetted. After about 30 seconds, the silver content of the solu tion to be examined, for example a fixing bath, is determined by comparison with a color scale. The color scale below gives the corresponding color shades obtained with the test reagent at various Ag(l) concentrations, (from yellow to brownish yellow to yellowish brown to brown).

COLOR SCALE Ag(l) cone. (g./l.) lndicator Color 7 0 lemon yellow 0.5 yellow 1.0 brownish yellow 1.4 yellowish brown 2.0 ochre 3.0 dark ochre 6 blackish brown 10 brownish black TABLE Solution 1 Solution 2 Example CdCl (g.) H SeO (g.) Na S(g.) Water:

methanol 1 (a) 0.3 0.08 0.3 1:0 (b) 0.65 0.08 0.6 1:0 (c) 1.3 0.08 1.2 1:0 (d) 1.95 0.08 1.8 1:0 (e) 2.6 0.08 2.4 1:0 (f) 5.0 0.08 4.9 1:0 (g) 0.3-5.0 0.01 0.3-4.9 1:0 (h) 0.3-5.0 0.04 0.3-4.9 1:0 (i) 0.3-5.0 0.1 0.3-4.9 1:0 (j) 0.3-5.0 0.2 0.3-4.9 1:0 (k) 0.3-5.0 0.5 0.3-4.9 1:0 (I) 1.95 0.08 2.3 1:0 (m) 2.6 0.08 6.0 1:0 (11) 1.3 0.08 1.2 1:5 (0) 1.3 0.08 1.2 1:7 (p) 1.3 0.08 1.2 1:9 (q) 1.3 0.08 1.2 1:10 (r) 1.3 0.08 1.2 1:20

EXAMPLE 2 Analogously to Example 1(d), the filter paper is impregnated with a solution 1 and 2, wherein solution 1 contains, as a substitute for H SeO an equivalent amount (0.107 g.) of Na SeO After immersion into a solution containing a brown concentration of silver ions 6g/l), a brown discoloration occurs.

EXAMPLE 3 20 g. of pure paper linters is whipped up in a solution of 1.5 g. of CdCl in l 1. of H 0. With vigorous agitation, a solution of 1.8 g. of Na s QH O in 50 mi. of water is added dropwise thereto. The yellow-orange colored pulp processed into paper on a paper machine having, after drying, a yellow color and a weight per unit area of about g./m".

Even with solutions having a silver ion content of 50 p.p.m., a marked darkening of the yellow color is obtained with the test paper.

EXAMPLE 4 A solution of 0.16 g. of Na S in 2.5 ml. of water is combined, with shaking, with 0.12 g. of Na SeO in 3 ml. of H 0.

By the addition of 0.6 ml. of 1N H 80 selenium is precipitated in a very finely dispersed form.

The thus-obtained selenium suspension is mixed with 1 g. of cadmium chloride (CdCl H O) in 5 ml. of H and mixed with 80 ml. of an aqueous 0.5 percent hydroxypropylcellulose solution. By the addition of 1.1 g..of Na s 91-1 0 in ml. of H 0 under agitation, cadmium sulfide is precipitated in a very finely dispersed form on the selenium which is finely and uniformly distributed in the water by the protective colloid.

The resulting CdS/Se suspension is highly suitable for the detection of silver ions in concentrations as low as p.p.m., e.g., fixing baths.

The suspension can also be diluted up to eight-fold, preferably with a 0.5 percent aqueons hydroxypropylcellulose solution. Even after such dilution, accurate determination of Ag ions is possible.

EXAMPLE 5 0.12 g. of Na SeO in 5 ml. of water is mixed dropwise with 0.1 g. of ascorbic acid in 2 ml. of water. The thus-obtained selenium suspension is then reacted with CdS analogously to Example 4. When a solution containing 1 g. of Ag /l. plus iron ions is added thereto a marked color change toward yellowish brown can be detected.

EXAMPLE 6 Test strips are prepared according to Example 1, except, instead of using CdCl another water-soluble cadmium salt set forth in the table below is employed to form the CdS suspen sion.

Otherwise Example Cd-Salt Amount Analogous to (g.) Example (:1) CdBr 0.88 1(b) (b) Cd(0OCCH 1.7 1(n) (c) Cd(NO )2 5.9 1(f) (d) CdSO 1.66 HQ) The preceding examples can be repeated with similar success by substituting the generically or specifically described reactants and/or operating conditions of this invention for those used in the preceding examples.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention,

and without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.

What is claimed is: g

' 1. A composition for the colorimetric detection of silver ions in afluid comprising an intimate, substantially homogeneous physical mixture of cadmium sulfide and particulate selenium.

2. A composition according to claim 1 wherein the selenium and cadmium sulfide are present in a weight ratio of 1:3 to about 1:75.

3. A composition according to claim 2 wherein the selenium and cadmium sulfide are present in a weight ratio of 1:8 to about 1:40.

4. A composition according to claim 1, wherein said cadmium sulfide is formed in the presence of finely dispersed seleni- 5. A composition according to claim 4 wherein the physical mixture is produced by reacting a water-soluble cadmium salt with a water-soluble sulfide in the presence of a suspension of finely divided selenium.

6. A composition according to claim 5 wherein the watersoluble cadmium salt is cadmium chloride.

7. A composition according to claim 5 wherein the physical mixture is formed by adding a solution of the sulfide to a solution of a mixture of the water soluble cadmium salt and selenious acid or a water soluble salt thereof.

8. A composition according to claim 7 wherein the cadmium salt is cadmium chloride and the sulfide is sodiumsulfide.

9. An agent for the colorimetric detection of silver ions comprising an absorbent carrier impregnated with a composition according to claim 1.

10. An agent for the colorimetric detection of silver ions comprising an absorbent carrier impregnated with a composition according to claim 2.

11. An agent for the colorimetric detection of silver ions comprising an absorbent carrier impregnated with a composition according to claim 3.

12. An agent according to claim 9 wherein the absorbent carrier is paper.

13. An agent according to claim 12 wherein the paper has a weight per unit area of -140 g./m" and contains absorbed thereon about 0.45-9 grams of CdS and about 0.01-0.8 grams of selenium per m 

2. A composition according to claim 1 wherein the selenium and cadmium sulfide are present in a weight ratio of 1:3 to about 1:
 75. 3. A composition according to claim 2 wherein the selenium and cadmium sulfide are present in a weight ratio of 1:8 to about 1:
 40. 4. A composition according to claim 1, wherein said cadmium sulfide is formed in the presence of finely dispersed selenium.
 5. A composition according to claim 4 wherein the physical mixture is produced by reacting a water-soluble cadmium salt with a water-soluble sulfide in the presence of a suspension of finely divided selenium.
 6. A composition according to claim 5 wherein the water-soluble cadmium salt is cadmium chloride.
 7. A composition according to claim 5 wherein the physical mixture is formed by adding a solution of the sulfide to a solution of a mixture of the water soluble cadmium salt and selenious acid or a water soluble salt thereof.
 8. A composition according to claim 7 wherein the cadmium salt is cadmium chloride and the sulfide is sodium sulfide.
 9. An agent for the colorimetric detection of silver ions comprising an absorbent carrier impregnated with a composition according to claim
 1. 10. An agent for the colorimetric detection of silver ions comprising an absorbent carrier impregnated with a composition according to claim
 2. 11. An agent for the colorimetric detection of silver ions comprising an absorbent carrier impregnated with a composition according to claim
 3. 12. An agent according to claim 9 wherein the absorbent carrier is paper.
 13. An agent according to claim 12 wherein the paper has a weight per unit area of 100-140 g./m2 and contains absorbed thereon about 0.45-9 grams of CdS and about 0.01-0.8 grams of selenium per m2. 